This invention relates to fan engineering, and more particularly to axial-flow fans (one-stage, two-stage and multi-stage fans).
The invention can find application in designing axial-flow fans intended for ventilation and having stable pressure characteristics within a wide range of air or gas delivery.
Axial-flow fans find multiple applications in industry. A factor which hampers wide use is the appearance of stall flow at the periphery of the rotor blades resulting in considerable drop in head and pressure oscillations at reduced rates of air or gas delivery.
There is known an axial-flow fan featuring a wide range of stable operation (cf.,e.g. U.S. Pat. No. 3,189,260) which comprises a stepped casing having inlet and discharge portions. The diameter of the discharge portion is substantially smaller than the diameter of the inlet portion and the inlet portion accommodates a ferrule to form between the casing and the ferrule an annular passage for by-passing air flow during flow stalls. Guide vanes are provided in the ferrule. The discharge portion of the casing accommodates a rotor with a set of blades, forward tips of the blades being disposed in the inlet portion of the housing.
However, in this known fan the above annular passage is excessively extended in the axial direction, which results in increased longitudinal dimensions of the fan.
In addition, the guide vanes provided in the ferrule to straighten stalled flows increase aerodynamic resistance at the inlet portion of the casing before the rotor resulting in reduced head and efficiency of the fan.
Also known are axial-flow fans comprising a stepped housing having in the discharge portion thereof a rotor carrying a plurality of blades, the inlet portion of the housing including a ferrule defining with the housing an annular passage wherein there are affixed guide vanes to straighten the flow (cf., e.g., the West German Magazine H. D. Henssler Neue Axialventilatoren ohne instabilen Betriebsbereich, VGB Kraftwerkstechnik, 57 Jahrgang, Heft 3, Marz 1977, Seiten 159 bis 165).
In these fans the stalled flow at the outlet from the annular passage tends to partially block the inlet area to thereby hamper the passage of the main flow entering a flow-through zone before the rotor blades. Therefore, the amount of air delivered and the pressure in the flow stall areas are considerably reduced causing unstable operation.
There is further known an axial-flow fan generally similar to the one heretofore described and having guide vanes in an annular passage (cf. the pamphlet of "Licensintorg" "General-purpose Axial-flow Fan", Moscow, USSR).
An inlet tube is fixed to the inlet portion of the casing of the above fan, this inlet tube having an inner diameter substantially equal to the diameter of the diameter of the discharge portion of the casing and the guide vanes provided in the annular passage are confined by the axial dimensions of a ferrule.
The provision of the inlet tube improves to a certain extent the entry of the main air flow to the fan. However, due to the axially limited dimensions of the guide vanes, the stalled flow tends to retain a swirl conforming in direction to the direction of rotation of the rotor and, while exiting a flow-through area before the rotor blades, to hamper the stable passage of the main air flow. This causes reduced head and operating stability of the fan in flow stall conditions.
In view of the aforegoing, the provision of guide vanes in the annular passage in the known axial-flow fans fails to optimize head in flow stall conditions.